This invention relates to a process for the production of chlorine dioxide.
Chlorine dioxide is utilized in a variety of processes, including a large number of bactericidal applications, especially in the fields of water treatment and odor abatement. Its usage is continuing to grow for many reasons. Due to the unstable nature of gaseous chlorine dioxide when compressed, it is necessary to produce chlorine dioxide on site rather than to produce it at a plant and ship it for usage when and where needed.
The prior art discloses a variety of different types of chlorine dioxide generators, most of which comprise elongated glass or plastic columns of various constructions. Reference is made, for instance, to U.S. Pat. No. 4,013,761 and to U.S. Pat. No. 3,502,443.
All of the prior art chlorine dioxide generators which employ columns tend to be large and bulky. They require the use of chemical feed pumps to meter in the raw materials utilized for the process. One of the problems associated with such prior art chlorine dioxide generators is the inadequacy of the mixing to achieve reaction of the raw materials being employed to produce chlorine dioxide.
My U.S. Pat. No. 4,247,531 titled "Chlorine Dioxide Generation Apparatus and Process" discloses apparatus and process whereby chlorine dioxide is generated by the essentially instantaneous reaction of a relatively concentrated aqueous solution of chlorine dioxide precursors, preferably a metal chlorite and an oxidizing agent which is preferably gaseous chlorine. This discovery has greatly simplified chlorine dioxide generation and reduced the size of the reactors while at the same time producing high yields of chlorine dioxide. The reaction is carried out in a small generator having a mix and dwell premix chamber interconnected with a source of vacuum, preferably a venturi. The relatively concentrated aqueous metal chlorite solution and the oxidizing agent are allowed to mix and react in the premix chamber prior to any dilution as, for example, by water or other fluid flowing through the venturi eductor to deliver the generated chlorine dioxide to storage or utilization. In order to achieve the essentially instantaneous reaction of the relatively concentrated aqueous solution of metal chlorite and gaseous chlorine, it is critical that the reactants, either before entering the reaction zone or while in the reaction zone, not be diluted by any diluent, for example water, in an amount sufficient to have a quenching effect on the reaction. At column 9, lines 40-48 of U.S. Pat. No. 4,247,531, it is taught that when the aqueous metal chlorite solution has a concentration above about 10%, or even when diluted to somewhat less than 10%, excellent results are obtained. It is important that the metal chlorite solution, initially and in the reaction zone, be more concentrated than the approximately 1% aqueous metal chlorite solutions of the prior art.
When the relatively concentrated aqueous metal chlorite solution and gaseous chlorine are reacted in the chlorine dioxide generator described in U.S. Pat. No. 4,247,531, there is a tendency for deposits of solid metal chloride to build up on the chlorine inlet to the premix chamber of the generator. As shown in the equation below, sodium chloride is a by product of the reaction of a metal chlorite and chlorine. Any aqueous metal chlorite in the vicinity of the in-rushing dry chlorine gas would be reduced to metal chloride. Because of the dry gas entering the reactor at relatively high velocity, the metal chloride is dried into crystals on the chlorine inlet that eventually block the free flow of the gaseous chlorine. Accordingly, in the practice of this invention, the gaseous chlorine stream contains some bleed water in an amount below that which would significantly dilute the reactants and thereby have a quenching effect on the reaction. The presence of bleed water prevents a buildup of metal chloride on the chlorine inlet to the premix chamber.